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Ordovician:
– Brachiopods were extremely abundant during this phase of evolution still largely dominated by invertebrates. Brachiopods are animals with a shell whose soft parts are contained between two valves. Although they resemble bivalves like clams, the body parts inside the shell are very different. Additionally, their valves have a different symmetry than clams. A pedicle emerges from the larger of the two valves, allowing this brachiopod to attach itself to the seafloor. Brachiopods are protostomes, one of two extant groups of bilaterians, along with bryozoans. Bryozoans are mossy animals that live in colonies. They take various forms, including the tree or lace fan. Although they look suspiciously like corals, they are not corals.
Each individual secretes an arborescent skeleton of calcium carbonate attached to a hard surface. Each specimen, called a zooid, is generally less than 1 millimeter in length and lives in its own small chamber. All interconnected, they form colonies containing millions of zooids, which can reach several meters. Within the same colony, each individual plays a distinct role. Some gather food, others help strengthen the group. Finally, some of these people contribute to the household. To feed, bryozoans create a stream of water that brings tiny creatures to their mouths. Most of the time, these are plant organisms such as phytoplankton. Bryozoans gradually colonize certain areas of the seabed, eventually forming vast reefs.
Trilobites make up a large part of the Ordovician fauna. These crustaceans, which secreted an easily identifiable segmented exoskeleton, reached their peak during the Ordovician. In fact, they are the most famous prehistoric animals after the dinosaurs. More than 18,000 species have been recorded. They ranged in size from 1 to 70 cm in length, and their lifestyle spanned the full range of thalassic environments. Some were detritivores, grazers or even predators. While a few species mastered swimming, the majority of trilobites moved along the seafloor, burying themselves in sediment to escape predators. Trilobites also developed the first advanced visual system. All trilobites had a head, called a cephalon, a thorax and a tail, as well as pairs of biramous legs on the ventral side. As in crustaceans, these legs had two branches, one serving as gills for breathing, the other for locomotion and feeding. The most common trilobites include Cryptolithus, which measured about 1.5 cm in length, and Acidaspis, almost 2 cm. But there were also specimens of much more spectacular dimensions. These included the Calymene, measuring between 2 and 5 cm long, and especially the Isoletus, which could commonly reach up to 38 cm long. However, the largest species of trilobite ever discovered in the world is Isoletus rex. Its complete fossil measures no less than 72 cm long and 40 cm wide.
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Today's program:
– 00:00 – Presentation
– 01:54 – After the Cambrian-Ordovician extinction
– 06:57 – The continents in the Ordovician era
– 10:39 – Climate during the Ordovician period
– 2:40 p.m. – Life spreads in the oceans
– 5:00 p.m. – Life on the continents
– 6:51 p.m. – First known Lagerstätten
– 22:16 – Fezouata in Morocco
– 24h48 – Taichoute site
– 27:29 – Recent discovery in England
– 30:19 – Incredible creatures from the Ordovician period
– 49:46 – Evolution of species
– 51:51 – Ordovician crisis
– 01:20:15 – What's next?
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